ITER operation beyond its baseline scenario

The `improved H-mode' regime, realized in ASDEX Upgrade in 1998 and confirmed by other devices, demonstrates the integration of advanced performance beyond the standard H-mode for confinement (confinement enhancement factor H-98(y,H-2) <= 1.6), stability (normalized beta beta N similar to 3-3.5) and, at densities close to the Greenwald density, exhaust in stationary discharges longer than 40 confinement times or up to six resistive diffusion times. The q-profile is characterized by low central magnetic shear and axis safety factor q0 > 1 that is obtained by particular discharge and heating ramp-up scenarios and maintained via fishbones or benign higher (m, n) instabilities without using elaborate current control. Core transport is still governed by driftwave turbulence with stiff temperature profiles, but density profiles are more strongly peaked and contribute to the increase in global confinement. A further contribution manifests itself by enhanced pressures at the edge barrier pedestal top and at the rho = 0.9 surface both increasing with the input power. (3, 2) NTMs remain small, enabling routine operation up to beta N similar to 3 (limited by (2, 1) NTMs) at ITER relevant collisionalities, for normalized Larmor radii down to four times the ITER value and for a broad range of q(95) = 3-5. Tailored heat deposition including central wave heating allows for a compromise in density peaking for enhanced confinement and tolerable high-Z impurity concentrations even with tungsten coated structures. As far as the ITER relevance of this regime is concerned, its compatibility with significant central electron heating, low collisionality and even densities close to the Greenwald density combined with type-II ELMS and,beta N similar to 3.5 is of importance. The GLF23 turbulence model still predicts peaked density profiles (R/L-n similar to 3) and sufficient transport to avoid impurity accumulation. At low q(95) similar to 3 the fusion performance in terms of beta N (.) H-98(y,H-2)/(q) (2)(95) is more than doubled compared with the ITER baseline scenario (performance factor similar to 0.2) extrapolating to long >> 10 pulses on ITER. At medium q's bootstrap current fractions up to 50% and performance factors close to 0.2 can be achieved resulting in long inductive pulse lengths of similar to 1 h allowing ITER `hybrid' operation at Q <= 9.